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Microbe-Human Interactions. The human body exists in a state of dynamic equilibrium Many interactions between human body and microorganisms involve the development of biofilms Colonization of the body involves a constant “give and take”. Contact, Colonization, Infection, Disease.

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Microbe-Human Interactions

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microbe human interactions
Microbe-Human Interactions
  • The human body exists in a state of dynamic equilibrium
  • Many interactions between human body and microorganisms involve the development of biofilms
  • Colonization of the body involves a constant “give and take”
contact colonization infection disease
Contact, Colonization, Infection, Disease
  • Microbes that engage in mutual or commensal associations – normal (resident) flora, indigenous flora, microbiota
  • Infection – a condition in which pathogenic microbes penetrate host defenses, enter tissues, and multiply
  • Pathogen – infectious agent
  • Infectious disease – an infection that causes damage or disruption to tissues and organs
resident flora
Resident Flora
  • Most areas of the body in contact with the outside environment harbor resident microbes
  • Internal organs, tissues, and fluids are microbe-free
  • Transients – microbes that occupy the body for only short periods
  • Residents – microbes that become established
resident flora7
Resident Flora
  • Bacterial flora benefit host by preventing overgrowth of harmful microbes – microbial antagonism
  • Endogenous infections – occur when normal flora is introduced to a site that was previously sterile
initial colonization of the newborn
Initial Colonization of the Newborn
  • Uterus and contents are normally sterile and remain so until just before birth
  • Breaking of fetal membrane exposes the infant; all subsequent handling and feeding continue to introduce what will be normal flora
flora of the human skin
Flora of the Human Skin
  • Skin is the largest and most accessible organ
  • Two cutaneous populations
    • Transients: influenced by hygiene
    • Resident: stable, predictable, less influenced by hygiene
flora of the gastrointestinal tract
Flora of the Gastrointestinal Tract
  • GI tract is a long hollow tube, bounded by mucous membranes
    • Tube is exposed to the environment
  • Variations in flora distribution due to shifting conditions (pH, oxygen tension, anatomy)
  • Oral cavity, large intestine, and rectum harbor appreciable flora
flora of the mouth
Flora of the Mouth
  • Most diverse and unique flora of the body
  • Numerous adaptive niches
  • Bacterial count of saliva (5 x 109 cells per milliliter)
flora of the large intestine
Flora of the Large Intestine
  • Has complex and profound interactions with host
  • 108-1011 microbes per gram of feces
  • Intestinal environment favors anaerobic bacteria
  • Intestinal bacteria contribute to intestinal odor
flora of the respiratory tract
Flora of the Respiratory Tract
  • Oral streptococci, first organisms to colonize
  • Nasal entrance, nasal vestibule, anterior nasopharynx – S. aureus
  • Mucous membranes of nasopharynx – Neisseria
  • Tonsils and lower pharynx – Haemophilus
flora of the genitourinary tract
Flora of the Genitourinary Tract
  • Sites that harbor microflora
    • Females – Vagina and outer opening of urethra
    • Males – Anterior urethra
  • Changes in physiology influence the composition of the normal flora
    • Vagina (estrogen, glycogen, pH)
maintenance of the normal resident flora
Maintenance of the Normal Resident Flora
  • Normal flora is essential to the health of humans
  • Flora create an environment that may prevent infections and can enhance host defenses
  • Antibiotics, dietary changes, and disease may alter flora
  • Probiotics – introducing known microbes back into the body
True pathogens – capable of causing disease in healthy persons with normal immune defenses
    • Influenza virus, plague bacillus, malarial protozoan
  • Opportunistic pathogens – cause disease when the host’s defenses are compromised or when they grow in part of the body that is not natural to them
    • Pseudomonas sp & Candida albicans
  • Severity of the disease depends on the virulence of the pathogen; characteristic or structure that contributes to the ability of a microbe to cause disease is a virulence factor.
becoming established
Becoming Established

Portals of entry – characteristic route a microbe follows to enter the tissues of the body

    • Skin – nicks, abrasions, punctures, incisions
    • Gastrointestinal tract – food, drink, and other ingested materials
    • Respiratory tract – oral and nasal cavities
    • Urogenital tract – sexual, displaced organisms
    • Transplacental
  • Exogenous agents originate from source outside the body
  • Endogenous agents already exist on or in the body (normal flora)
requirement for an infectious dose id
Requirement for an Infectious Dose (ID)
  • Minimum number of microbes required for infection to proceed
  • Microbes with small IDs have greater virulence
  • Lack of ID will not result in infection
attaching to the host
Attaching to the Host
  • Adhesion – microbes gain a stable foothold at the portal of entry; dependent on binding between specific molecules on host and pathogen
    • Fimbrae
    • Flagella
    • Adhesive slimes or capsules
    • Cilia
    • Suckers
    • Hooks
    • Barbs
surviving host defenses
Surviving Host Defenses
  • Initial response of host defenses comes from phagocytes
  • Antiphagocytic factors – used to avoid phagocytosis
  • Species of Staphylococcus and Streptococcus produce leukocidins, toxic to white blood cells
  • Slime layer or capsule – makes phagocytosis difficult
  • Ability to survive intracellular phagocytosis
causing disease
Causing Disease
  • Virulence factors – traits used to invade and establish themselves in the host, also determine the degree of tissue damage that occurs – severity of disease
  • Exoenzymes – dissolve extracellular barriers and penetrate through or between cells
  • Toxigenicity – capacity to produce toxins at the site of multiplication
    • Endotoxin – toxin that is not secreted but is released after the cell is damaged
    • Exotoxin – toxin molecule secreted by a living bacterial cell into the infected tissue
  • Antiphagocytic factors
bacterial toxins a potent source of cellular damage
Bacterial Toxins: A Potent Source of Cellular Damage
  • Exotoxins: Strong specificity for a target cell
    • Hemolysins
    • A-B toxins (A-active, B-binding)
  • Endotoxin – lipopolysaccharide (LPS), part of the outer membrane of gram-negative cell walls
the process of infection and disease
The Process of Infection and Disease
  • 4 distinct stages of clinical infections:
    • Incubation period – time from initial contact with the infectious agent to the appearance of first symptoms; agent is multiplying but damage is insufficient to cause symptoms; several hours to several years
    • Prodromal stage – vague feelings of discomfort; nonspecific complaints
    • Period of invasion – multiplies at high levels, becomes well-established; more specific signs and symptoms
    • Convalescent period –as person begins to respond to the infection, symptoms decline
establishment spread and pathologic effects
Establishment, Spread, and Pathologic Effects

Patterns of infection:

  • Localized infection – microbes enter the body and remains confined to a specific tissue
  • Systemic infection – infection spreads to several sites and tissue fluids usually in the bloodstream
  • Focal infection – when infectious agent breaks loose from a local infection and is carried to other tissues
patterns of infection
Patterns of Infection
  • Mixed infection – several microbes grow simultaneously at the infection site - polymicrobial
  • Primary infection – initial infection
  • Secondary infection – another infection by a different microbe
  • Acute infection – comes on rapidly, with severe but short-lived effects
  • Chronic infections – progress and persist over a long period of time
signs and symptoms of inflammation
Signs and Symptoms of Inflammation
  • Earliest symptoms of disease as a result of the activation of the body defenses
    • Fever, pain, soreness, swelling
  • Signs of inflammation:
    • Edema – accumulation of fluid
    • Granulomas and abscesses – walled-off collections of inflammatory cells and microbes
    • Lymphadenitis – swollen lymph nodes
signs of infection in the blood
Signs of Infection in the Blood
  • Changes in the number of circulating white blood cells
    • Leukocytosis – increase in white blood cells
    • Leukopenia – decrease in white blood cells
    • Septicemia – microorganisms are multiplying in the blood and present in large numbers
      • Bacteremia – small numbers of bacteria present in blood not necessarily multiplying
      • Viremia – small number of viruses present not necessarily multiplying
infections that go unnoticed
Infections That Go Unnoticed
  • Asymptomatic (subclinical) infections – although infected, the host doesn’t show any signs of disease
  • Inapparent infection, so person doesn’t seek medical attention
portals of exit
Portals of Exit
  • Pathogens depart by a specific avenue; greatly influences the dissemination of infection
    • Respiratory – mucus, sputum, nasal drainage, saliva
    • Skin scales
    • Fecal exit
    • Urogenital tract
    • Removal of blood
persistence of microbes and pathologic conditions
Persistence of Microbes and Pathologic Conditions
  • Apparent recovery of host does not always mean the microbe has been removed
  • Latency – after the initial symptoms in certain chronic diseases, the microbe can periodically become active and produce a recurrent disease; person may or may not shed it during the latent stage
  • Chronic carrier – person with a latent infection who sheds the infectious agent
  • Sequelae – long-term or permanent damage to tissues or organs
sources and transmission of microbes
Sources and Transmission of Microbes
  • Reservoir – primary habitat of pathogen in the natural world
    • Human or animal carrier, soil, water, plants
  • Source – individual or object from which an infection is actually acquired
living reservoirs
Living Reservoirs
  • Carrier – an individual who inconspicuously shelters a pathogen and spreads it to others; may or may not have experienced disease due to the microbe
  • Asymptomatic carrier – show no symptoms
    • Incubation carriers – spread the infectious agent during the incubation period
    • Convalescent carriers – recuperating without symptoms
    • Chronic carrier – individual who shelters the infectious agent for a long period
  • Passive carrier – contaminated healthcare provider picks up pathogens and transfers them to other patients
animals as reservoirs and sources
Animals as Reservoirs and Sources
  • A live animal (other than human) that transmits an infectious agent from one host to another is called a vector
  • Majority of vectors are arthropods – fleas, mosquitoes, flies, and ticks
  • Some larger animals can also spread infection – mammals, birds, lower vertebrates
  • Biological vectors –actively participate in a pathogen’s life cycle
  • Mechanical vector – not necessary to the life cycle of an infectious agent and merely transports it without being infected
An infection indigenous to animals but naturally transmissible to humans is a zoonosis
  • Humans don’t transmit the disease to others
  • At least 150 zoonoses exist worldwide; make up 70% of all new emerging diseases worldwide
  • Impossible to eradicate the disease without eradicating the animal reservoir
acquisition and transmission of infectious agents
Acquisition and Transmission of Infectious Agents
  • Communicable disease – when an infected host can transmit the infectious agent to another host and establish infection in that host
  • Highly communicable disease is contagious
  • Non-communicable infectious disease does not arise through transmission from host to host
    • Occurs primarily when a compromised person is invaded by his or her own normal microflora
    • Contact with organism in natural, non-living reservoir
patterns of transmission
Patterns of Transmission
  • Direct contact – physical contact or fine aerosol droplets
  • Indirect contact – passes from infected host to intermediate conveyor and then to another host
    • Vehicle – inanimate material, food, water, biological products, fomites
    • Airborne – droplet nuclei, aerosols
nosocomial infections
Nosocomial Infections
  • Diseases that are acquired or developed during a hospital stay
  • From surgical procedures, equipment, personnel, and exposure to drug-resistant microorganisms
  • 2 to 4 million cases/year in U.S. with approximately 90,000 deaths
  • Most commonly involve urinary tract, respiratory tract, and surgical incisions
  • Most common organisms involved: Gram-negative intestinal flora
    • E. coli, Pseudomonas, Staphylococcus
universal blood and body fluid precautions
Universal Blood and Body Fluid Precautions
  • Stringent measures to prevent the spread of nosocomial infections from patient to patient, from patient to worker, and from worker to patient – universal precautions
  • Based on the assumption that all patient specimens could harbor infectious agents, so must be treated with the same degree of care
  • The study of the frequency and distribution of disease and health-related factors in human populations
  • Surveillance – collecting, analyzing, and reporting data on rates of occurrence, mortality, morbidity and transmission of infections
  • Reportable, notifiable diseases must be reported to authorities
Centers for Disease Control and Prevention (CDC) in Atlanta, GA – principal government agency responsible for keeping track of infectious diseases nationwide
  • http://www.cdc.gov
  • Morbidity and Mortality Weekly Report


frequency of cases
Frequency of Cases
  • Prevalence – total number of existing cases with respect to the entire population usually represented by a percentage of the population
  • Incidence – measures the number of new cases over a certain time period, as compared with the general healthy population
Mortality rate – the total number of deaths in a population due to a certain disease
  • Morbidity rate – number of people afflicted with a certain disease
Endemic – disease that exhibits a relatively steady frequency over a long period of time in a particular geographic locale
  • Sporadic – when occasional cases are reported at irregular intervals
  • Epidemic – when prevalence of a disease is increasing beyond what is expected
  • Pandemic – epidemic across continents
koch s postulates
Koch’s Postulates

Determining the causative or etiologic agent of infectious disease:

  • Find evidence of a particular microbe in every case of a disease
  • Isolate that microbe from an infected subject and cultivate it artificially in the laboratory
  • Inoculate a susceptible healthy subject with the laboratory isolate and observe the resultant disease
  • Reisolate the agent from this subject